They have complicated control computers and are very touchy concerning the amount of voltage in various parts of the system, such as how much current the motor is drawing. Sometimes transmissions go dry of oil causing the gears to bind, throttles develop oxidation on the contacts, terminals get corroded. I had to bypass my original unreliable controller completely with a DC variable speed board from China. Test your batteries by connecting each to their own standard car charger. Get the back wheels off the ground with blocks and test run the motor while connected to the chargers. And test each battery with something powerful that can determine its strength, not just a small test light.
The chargers are reliable, but the control brain (module) maybe not. Make sure your batteries are not dry, make sure fuse is OK, terminals are clean, charging system must be unplugged to run, test test test. Make sure the speed limiter is tried in various positions, parking brake or lock is in correct position.